A certain class of sensors configured into an array of pixilated capacitive elements make use of ferroelectric materials, and their pyroelectric effect for detection of temperature change. Sensors of this type have a wide range of applications, such as imaging in low visibility conditions, for example, poor weather conditions, night vision, etc. A ferroelectric material is a dielectric material that has a temperature dependent spontaneous electrical polarization. Each pixel element is thermally sensitive to a portion of a scene to be imaged. The material is thermally biased and referenced by a chopper so that the added or diminished IR energy from the scene either raises or lowers the temperature of the ferroelectric material from a reference temperature preferably at room temperature. This in turn, changes the polarization of the ferroelectric material which is sensed as a momentary current change or alternatively, a voltage change when the change state of a reference capacitor is charged or discharged, dependent upon the temperature of the pixel element. IR imaging is permitted because the individual pixel elements vary their local polarization state due to the IR radiation imparted to them from the scene. The temperature is then reset to the reference temperature via thermal cooling or heating when the chopper is closed isolating the pixel element from the scene.
Generally, known ferroelectric/pyroelectric sensors that convert varying radiation energy to usable electrical signals greater than the inherent ambient noise of the sensor system operate in a “passive mode.” This means that the pyroelectric element operates at a given polarization state which is a function of temperature change, without any deliberate electrical polarization reversal (as best shown in FIG. 1). More specifically, passive pyroelectric detection only interrogates the polarization state of the ferroelectric material typically by measuring the net voltage, ΔQ, across a poled capacitor structure wherein ΔQ=ΔP*A and A is the element area, or by small signal AC excitation to determine the permittivity of the material (which is a function of the poled state), or some combination of these two methods.
The practice in the industry to compare ferroelectric/pyroelectric sensors has been to measure the pyroelectric coefficient p, which is defined as the partial derivative of the displacement D with respect to the temperature T, p=(ΔD/ΔT) at a given bias field, Eb, noting, that D=∈E+P wherein ∈ is a dielectric constant. What this means is that for a physical geometry having sensor area, A, the amount of coulombs of charge, Q, is generated per temperature, T, and the pyroelectric coefficient, p, is expressed as: p=(1/A) [ΔQ/ΔT]. Unfortunately, this technique only represents a single cycle around a minor portion of the available signal energy as represented by the hysteresis loop area of FIG. 2.
FIG. 3 shows a schematic block diagram of a known pyroelectric sensor system 10 that employs a conventional passive charge generation technique to determine the output of the sensor element. The sensor system 10 includes a chopper 12 that selectively gates radiation from a scene onto an infrared absorber 14 that is part of a pyroelectric element 16. The pyroelectric element 16 is made of a ferroelectric material that exhibits hysteresis loops which vary with temperature as shown in FIG. 2, and represents a single pixel element of the sensor system 10 that combines with other pixel elements (not shown) to generate an image, as is well understood in the art. The discussion herein is directed to an infrared imaging system, but as will be appreciated by those skilled in the art, sensor systems of this type are applicable to detect other wavelengths of radiation, including millimeter waves and microwaves.
The chopper 12 selectively blocks and passes the radiation directed to the pyroelectric element 16 at a predetermined frequency so that the pyroelectric element 16 sees a reference temperature when the chopper 12 is closed, and sees the temperature of the scene when the chopper 12 is open. The difference between the reference temperature and the scene temperature alters the shape of the hysteresis loop, as shown in FIG. 2. The change in charge Q(t) 18 for the two loops is measured separately as a voltage across a sampling or output capacitor 20 and amplifier 22, in a manner that is well understood in the art. Because no external electric field is applied to the pyroelectric element 16, the measure charge of the pyroelectric element 16 that charges the capacitor 20 for the two loops is the charge Q1 where the hysteresis loop crosses the positive vertical axis for temperature T1 and the charge Q2 where the hysteresis loop crosses the positive vertical axis for temperature T2. The sampling capacitor 20 stores the charge from the pyroelectric element 16 only each time the window is opened by the chopper 12. The effective pyroelectric coefficient p for this design is given as:p=(1/A)[Q1−Q2]/[T1−T2]
In an alternate known design, the small signal level capacitance, (i.e. change in local slope of the Q versus V curve of either a poled or unpoled ferroelectric material) between the charge stored by the pyroelectric element 16 is measured for temperature T1 and T2 and then compared. FIG. 4 shows a schematic block diagram of a sensor system 26 including the chopper 12, the infrared absorber 14, the pyroelectric element 16 and the amplifier 22. Sometimes a small bias voltage is applied to the pyroelectric element 16 from a bias source (not shown), and a capacitance meter 28 is used to measure the change in capacitance between the location on the hysteresis loop for both temperatures T1 and T2 relative to the bias voltage. Even though a small bias voltage is applied to the pyroelectric element 16 in this design, the mode of operations is still passive because the small bias voltage does not alter the polarization state of the ferroelectric material in any way, but merely measures its change in local permittivity as measured by a change in capacitance. The effective pyroelectric coefficient p is given as:p=[(Vmns)/A](ΔC/ΔT)
As is apparent, this detection scheme utilizes only a small portion of the hysteresis loop, and therefore the sensors are limited in their ability to differentiate signal from noise. Both of the techniques discussed above are dependent upon the condition that the ferroelectric material is left resident in one of its two spontaneous polarization states PS (+ or −), or some intermediate state thereof. The ability to measure the power from the pyroelectric element 16 between the temperature changes gives the sensitivity of the system. Because the signal-to-noise ratio is relatively low for the prior art sensors, this establishes the sensitivity of the entire system. Robust and relatively expensive system components, such as the chopper 12 and the amplifier 22 cannot increase the signal from noise, but only prevents further degradation.
As illustrated in FIG. 2, the polarization magnitude and direction within the ferroelectric material is identifiable by a hysteresis loop. The orientation of the polarization of the material can be changed by applying a reversing external electric field to the material. The electric dipoles within the material, that identify the orientation of the polarization, change when the external field is applied and in proper circuit layout produce a hysteresis loop. Since spontaneous polarization is generally temperature dependent, ferroelectric materials can employ the pyroelectric effect for temperature detection purposes.
Any area of the hysteresis loop, either the entire saturated hysteresis area or merely a region of operation anywhere within the full loop, is representative of the switching energy required to change the polarization states of some or all the dipoles which make up the atomic lattice structure of the material at a given temperature for the specific state of excitation. Any change in radiation incident of the ferroelectric material, if absorbed, changes the temperature, and thus changes the associate loop area. FIG. 2 shows two charges versus voltage hysteresis loops for a particular pyroelectric material at a first temperature T1 and a second temperature T2. If plotted independent of physical dimensions, the magnitude of an externally applied alternating electric field is given on the horizontal axis and polarization, in charge density, is given on the vertical axis. The area of the charge versus voltage hysteresis loop of a ferroelectric material has dimensions of energy, and the loop area is a direct function of its temperature. The magnitude of the polarization changes with a change in the temperature of the pyroelectric material for a given electric field. A careful review of the two hysteresis loops in FIG. 2 will show that for the two different temperatures T1 and T2 (with T1<T2), the area within the loop is different. Consequently, an electrical measurement of the change in area anywhere within the major loop is an electrical signal corresponding to the change of the temperature of the material, and thus of the incident infrared radiation. The effect is of a dynamic nature due to the switching between polarization states of the pyroelectric material, and therefore, when measuring incident radiation, it is necessary to shutter the radiation, to reference the ferroelectric spontaneous polarization before each window opens to the scene.
More recent developments in pyroelectric technology is taught in U.S. Pat. No. 6,294,784 B1, filed Feb. 1, 1999, and U.S. Pat. No. 6,339,221 B1 filed Dec. 3, 1999, both being incorporated herein by reference. These patent disclose an “active” mode of operation as oppose to the traditional “passive” mode previously described. In the active mode, the individual elements are driven by an external voltage to switch between its positive and negative polarization states at a voltage level sufficient to get a significant displacement switching current.
In active mode, each time the polarization state switches, a charge, Qs, equal to:Qs=Pr*A will be supplied from this external power source. When the amount of charge delivered is measured, rectified, integrated and amplified via a charge amplifier/integrator 48 of FIG. 6, as taught in the above referenced patents, the total charge accumulated for a preset time period, τ, is (without the amplification factor):Qtotal=(2*Qs)*f*τThe product of frequency, f, of the external power source times time, (f*τ) is equal to the number of switches, N. As the polarization state is determined by the temperature, Qtotal is a direct function of temperature. The output signal for each pixel element is the difference between a reference Qref and Qtotal taken after the time interval τ, wherein τ is the duration the chopper is open.
Because the polarization state is sampled multiple times the effective sensitivity of a pyroelectric material is enhanced by a factor of N and the signal to noise ratio averages out random noise, with a reduction factor for the noise component of 1/(f*τ)1/2. Unfortunately, to avoid saturation of the signal output voltage, Vo, an integrating or output capacitor 52 and 58 must be large enough to handle the total summed charge during the time τ. Thus the capacitance of capacitor 52 must be greater than the product of N times the inherent capacitance of the pyroelectric element 34. Utilizing an output capacitor of sufficient capacitance limits the ability to place the capacitor on the chip and causes the focal point array and supporting circuitry to be larger than desired.
Because material sensitivity is portrayed as the pyroelectric coefficient, p:p=dPs/dT the reference temperature required to promote sensitivity is best illustrated in FIG. 1 and is located at or near an abrupt change in spontaneous polarization, Ps, over a narrow temperature range. Such a temperature is characteristic of the material and is otherwise termed a curie temperature, Tc. Curie temperatures are customarily obtained from a plot of the inverse permittivity as a function of temperature and is the high temperature extrapolation to zero inverse permittivity, which represents a phase transition in the material from non-ferroelectric to ferroelectric. Traditionally, the reference temperature, Tref, is room temperature, hence, if the curie temperature, Tc, is some distance away from room temperature the slope of the line of FIG. 1 designating polarization change decreases which undesirably decreases sensitivity and the pyroelectric coefficient, p.
Within the art of pyroelectrics, a variety of materials are known having curie temperatures, which if used as the reference temperature, can be compatible with supporting structure of any variety of applications, and with a pyroelectric coefficient, p, can meet the desired sensitivity. The tables of known materials referenced from ISAF “92: Proceedings of the Eighth IEEE International Symposium on applications of ferroelectrics, p. 1, are:
Normal Pyroelectrics (T < Tc)MaterialTc° C.P μC/cm2KSingle crystalsTGS490.028DTGS600.055ATGSAs510.07LiTaO36650.18LiNbO312100.083SBN 46/541320.043PGO:Ba700.032CeramicsPLZT 7/65/351500.13PLZT 8/65/351050.18PZNFTU2300.039PSZNFTU1700.049PGO1780.002PolymericsPVDFNone0.0027Thin FilmsPbTiO3 sol-gel4900.0095PLT 90/10 sputtered3300.065PCT 70/30 sputtered2700.05PZT 54/46 sol-gel3800.07Phase Transition Materials (T ≡ Tt(c))P′ maxMaterialTt° C.μC/cm2KSingle crystalsDTGEB741.4KTN 67/3348.0BST 65/3550.3CeramicsBST 67/332123.0BST 67/33226.3BST 67/33240.70BST 65/35290.10PMN:La400.085PScT400.38PZT 94/6*500.37PZNT 90/8/2*300.185PCT 70/30:9C0W 96/41063.0PLzT 8/60/40 (sic)1423.2PZN/BT/PT 80/10/10**125.93PZN/BT/PT 80/10/10852.9Thin FilmsPScT sputtered400.52PScT sol-gel400.30PScT MOCVD400.08KTN metalorganic4020.0
Unfortunately, very few of the known materials have curie temperatures near room temperature of approximately 22° C. Moreover, the materials listed above require high temperature processing and require expensive manufacturing processes; some such as BST are hard to grow, some contain lead which is environmentally unfriendly, and still others contain expensive scandium.
Referring to FIG. 5, because IR pyrometry entails temperature sensitive ferroelectric materials, a high thermal time constant representing a high level of thermal isolation of the pixel element is desirable. Any heat transference or cross-talk between pixels or thermal conduction or thermal shorting to the silicon substrate will only serve to lower the thermal time constant hence reduce sensitivity and signal-to-noise ratio. Traditionally, an air gap or bridge provides the necessary thermal isolation between the silicon substrate and a pixel element with only a minimal thermal isolator or spin-on-glass, SOG, connecting the element to the substrate.
Unfortunately, the air bridge concept requires both a silicon manufacturing process and a ceramic process. Moreover, the manufacturing process is expensive, produces a low yield of good arrays, requires inefficient single unit handling and is not capable of wafer-level fabrication. Yet further, the element is limited in size reduction, which is increasingly important when viewing that a typical focal plane array, FPA, typically can have an array of 512×512 pixel elements.